1. Field of the Invention
The present invention relates to a system for controlling a turbocharger mounted in an internal combustion engine, the turbocharger having a rotary electric machine mounted on the rotatable shaft thereof and operable as a motor-generator which functions as a motor or a generator.
2. Description of the Related Art
Since exhaust gases are high in temperature and pressure various methods and systems have been proposed for recovering the energy of exhaust gases emitted from internal combustion engines as drive power or electric power. One such exhaust energy recovery system, which is disclosed in Japanese Laid-Open Patent Publication No. 63(1988)-272909, has a rotary electric machine, operable as a motor-generator, mounted on a turbine shaft that can be driven by the energy of exhaust gases. Supercharging operation of the rotary electric machine and fuel supply are controlled on the basis of the load condition of the engine and the depression of the accelerator pedal.
FIG. 4 of the accompanying drawings schematically shows the exhaust energy recovery system disclosed in the above publication. An engine 1 has an exhaust pipe 1b and an intake pipe 1a which are connected respectively to a turbine 2b and a compressor 2a of a turbocharger 2. A rotary electric machine 3, operable as a motor-generator, is mounted on a rotatable turbine shaft 2c of the turbocharger 2.
A battery 5 is connected to a controller 6 which comprises a microcomputer and has a power electric device as an input/output circuit. Depending on signals from an accelerator pedal movement sensor 1c, an engine load sensor 1d, an engine rotation sensor 1e, and a boost pressure sensor 1h, the controller 6 controls the rotary electric machine 3 as a motor with the electric power from the battery 5 increasing the boost pressure and amount of supplied fuel, based on the operating conditions of the engine 1. Therefore, when a high load is imposed on the engine 1, the engine 1 can be controlled to produce an engine output power corresponding to the maximum amount of depression of the accelerator pedal. As shown in the flowchart of FIG. 5, if in step (a) the amount of depression of the accelerator pedal is maximum, control goes to step (b). There is established a speed control range L (rpm)-H (rpm) in which the rotary electric machine 3 is to operate as a motor. If in step (b) the rotational speed of the turbocharger 2 (i.e., the rotational speed of the rotary electric machine 3) is lower than the lower level L (rpm), then the electric current supplied from the battery 5 is maximized in step (c). If in step (b) the rotational speed of the turbocharger 2 is higher than the lower level L (rpm), then control goes to step (d) which determines whether it is lower than the upper level H (rpm). If in step (d) the rotational speed of the turbocharger 2 is lower, than the higher level H (rpm), but in step (e) increasing, then control goes to step (f) in which the electric current supplied from the battery 5 is reduced. If the rotational speed of the turbocharger 2 decreases in the step (e), then control goes to a step (g) in which the electric current supplied from the battery 5 remains unchanged. If in step (d) the rotational speed of the turbocharger 2 reaches the upper level H (rpm), the supplied electric current is discontinued or reduced by a certain level, so that the rotational speed of the turbocharger 2 will not exceed the upper level H (rpm).
As described above, if the rotational speed of the turbocharger is lower than the lower level, then the supplied current is maximized to rotate the turbocharger at high speed irrespective of the degree by which the rotational speed of the turbocharger is lower than the lower level, and if the rotational speed of the turbocharger reaches the high level, then the supplied current is reduced to rotate the turbocharger at low speed irrespective of the degree by which the rotational speed of the turbocharger has increased up to the high level. As a result of this repeated rotational speed control process, the rotational speed of the turbocharger tends to vary in a wide range as shown in FIG. 6. The rotational speed of the turbocharger is controlled unstably because it does not converge in a smaller range.